This patent application is co-pending with one related patent application entitled xe2x80x9cNON-INVASIVE METHOD OF DETERMINING ABSOLUTE INTRACRANIAL PRESSURExe2x80x9d (NASA Case No. LAR 16510-1), by the same inventors as this patent application.
1. Field of the Invention
This invention relates to determination of intracranial pressure. More specifically, the invention is a non-invasive method for determining the diastolic intracranial pressure in a patient.
2. Description of the Related Art
Bone tissue is the most rigid of all animal tissues. The skull bone surrounds and protects one""s cranial complex which includes the brain and cerebrospinal fluid (CSF) surrounding the brain. The human brain and the spinal cord are immersed in CSF which is continuously generated and reabsorbed by the body. The CSF is contained in a membrane covering the inside of the skull and the spinal cord which terminates in a sack located at the sacrum. The brain and the membrane containing the CSF also contain blood vessels, which are in direct communication with the CSF and add to the total volume of the cerebrospinal system. The blood volume in these blood vessels varies rhythmically with the heartbeat thereby causing corresponding oscillations in the intracranial pressure (ICP). The collective compliance (i.e., the ability to increase in volume with increasing pressure) of the skull and CSF is too small to accommodate the pressure regulation needed for proper circulation of blood within the brain and spinal cord. Hence, pressure within the cranial complex is controlled by the compliance of the brain""s venous bed in association with the creation and removal of CSF by specialized structures within the brain.
Pressure is regulated by rate of production of CSF by the choroid plexus, and rate of removal of cerebrospinal fluid by the arachnoid villi. These rates therefore play a crucial role in blood flow regulation, while also relating to disease and pathologies which can occur. A complex interaction between the blood vessels and ICP accomplishes the needed regulation of blood flow in brain tissue.
Substantial effort has been devoted to understanding the dynamics of pulsatile effects on ICP. Towards this end, many investigators have developed an xe2x80x9cequation of statexe2x80x9d which describes pressure and volume relationships in the cranial complex. While the various relationships differ, it is generally accepted that increases in diastolic ICP (i.e., increases in ICP occurring during the diastolic rhythm of one""s heartbeat) generate intracranial hypertension that affects the viability and function of the human brain.
Given the above, monitoring of diastolic ICP is of significant diagnostic and post-operative importance for patients with cranial injuries, pathologies or other conditions that may affect the pressure of the subarachnoidal fluid around the brain, and for patients who have undergone brain surgery. In general, ICP has traditionally been measured and monitored by means of a pressure sensor inserted through the skull into the brain. Usually a hole is drilled in the skull and a catheter with a pressure sensor is inserted into the brain fluid. This known procedure, while simple and accurate is not suitable for long-term monitoring because an open wound must be maintained in the skull. Antibiotics are only partially effective in treating cranial infections so the pressure sensor can only be left in place for two weeks or less.
Long-term monitoring of ICP is currently achieved by implanting a pressure sensor and transmitter into the brain. The ICP is thereafter monitored by means of a receiver located outside the skull. However, this solution is not preferred because it includes the risks associated with implanting anything in the brain, and because of the problems associated with providing power to an implanted transmitter.
A variety of non-invasive systems and/or methods of measuring relative changes in ICP have been described in each of U.S. patent application Ser. Nos. 09/459,384, 09/493,044, 10/094,023, and 10/121,932. However, none of these provide for the measurement or determination of a diastolic ICP.
Accordingly, it is an object of the present invention to provide a method of determining diastolic ICP in a non-invasive fashion.
Another object of the present invention is to provide a method of determining diastolic ICP that minimizes the number of procedures used.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a method is presented for determining diastolic intracranial pressure (ICP) in a patient. A first change in the length of any path across the skull of the patient caused by a known change in ICP is measured. This first change relative to the known change in ICP is indicative of an elasticity constant for the patient. Next, a second change in the length of the path across the patient""s skull occurring between systolic and diastolic portions of the patient""s heartbeat is measured. The patient""s diastolic ICP is a function of the elasticity constant for the path and the second change in the length of the path across the patient""s skull.